Which statement describes a non-defining relative clause?

• A. It provides essential information about the noun
• B. It adds extra information about the noun
• C. It defines the noun with a relative pronoun
• D. It must not be separated by commas

Answer: (B)

Non-defining relative clauses provide extra information about a noun and are set off by commas. They can be removed without changing which noun is being talked about, because the noun is already identified. For example: My sister, who lives in London, is visiting. Here, the clause adds extra detail, not essential to identify the sister. That’s why the best description is that it adds extra information about the noun.

The other statements describe ideas associated with defining (restrictive) clauses, which identify which noun is meant and do not use commas to attach the clause. It’s also incorrect to say such clauses must not be separated by commas, since non-defining clauses do require commas.